As is well known, various contents such as images, music, and programs are distributed on digital broadcasting and Internet from servers of service providers offering contents to receiving terminals of users. These contents contain one or a plurality of resources (such as data files, program files, or the like), and are classified broadly into stream-type and multimedia-type contents. The stream-type contents are video and audio-based contents, containing only stream resources such as MPEG2-TS. The multimedia-type contents contain a plurality of resources such as stream resources, multimedia resources, monomedia resources referred from multimedia resources and the like. The multimedia resources are, for example, HTML generally used on Web pages, BML generally used on digital broadcasting, or the like. The monomedia resources are, for example, MPEG used for moving images; GIF, JPEG and PNG used for still images; MNG used for animation; MPEG2-AAC used for audio; and the like. In the multimedia resources, textual information to be displayed on a screen; reference, to be added to the textual information and presented, to monomedia resources; hyperlink to other multimedia resources; and the like can be described.
In general, a service provider subjects copyrighted contents to encryption so as to allow only authorized users to view the contents. When a user desires to view contents which have been encrypted, the user obtains view licenses containing decryption keys issued by the service provider through paying a charge, providing personal information, or the like, and subjects the encrypted contents to decryption (see Patent document 1).
In principle, one view license is provided for one piece of content, but sometimes a plurality of view licenses are provided thereto. For example, it is desired to vary usage rules regarding a period during which viewing is allowed, limitation on resources on which viewing is allowed, and the like. Although this can be easily achieved by providing a plurality of servers with resources encrypted by varying keys corresponding to respective usage rules, large-capacity servers are required because the same resources have to be redundantly stored. In order to avoid wasting server resources, therefore, a method called sub-license, for example, has been proposed (see non-patent document 1).
In this sub-license method, a key used for encryption of a resource is doubly encrypted further by another common key. For example, consider a case where it is desired to set two usage rules for a given resource, i.e., periods of one or two weeks. In this case, a resource encrypted by a key A and the key A encrypted by a key B are stored in a server for first one week. A resource encrypted by a key C and the key C encrypted by the key B are stored for next one week. A user who wants to view the resource for one week obtains a view license containing the key A (or the key C), and subjects the resource to decryption by using the key A (or the key C). On the other hand, a user who wants to view the resource for two weeks obtains a view license including the key B, subjects the key A to decryption by using the key B, and subjects the resource to decryption by using the key A for the first one week; and subjects the key C to decryption by using the key B, and subjects the resource to decryption by using the key C for the next one week.    Patent document 1: Japanese Laid-Open Patent Publication No. 2002-342518    Non-patent document 1: “CONDITIONAL ACCESS SYSTEM SPECIFICATIONS FOR DIGITAL BROADCASTING”, p 15-25, ARIB STD-B25 4.1 version, by Association of Radio Industries and Businesses, Jun. 5, 2003